The following discussion of the background is merely provided to aid the reader in understanding the invention and is not admitted to describe or constitute prior art to the invention.
Phosphatidylinositol based signaling pathways play crucial roles in the regulation of cell processes at the plasma membrane and in the nucleus. Two components of such pathways include phosphatidylinositol phosphate kinases and the phosphoinositide second messengers generated by these kinases.
In mammalian cells, there are two types of phosphatidylinositol phosphate kinases: Type I and Type II. Both types generate phosphoinositide second messengers. There are at least three isoforms of type I phosphatidylinositol phosphate kinase (“PIPKI”) termed α, β, and γ. All are differentially expressed spatially and temporally, thereby providing a mechanism of control of second messenger generation. Of the three type I PIP kinases, only PIPKIα targets to nuclear speckles, structures within the nucleus of mammalian cells that are enriched in pre-messenger RNA splicing factors.
The Type I PIPKIs, including PIPKIα, are the major producers of a second messenger named phosphatidylinositol-4,5-bisphosphate (“PI4,5P2”). PI4,5P2 is a phospholipid which plays a role in the regulation of many cellular signaling pathways, and though it is maintained at relatively constant levels in cells, it is hypothesized that small local changes in the spatial and temporal synthesis of PI4,5P2 defines its role as a second messenger. PI4,5P2 is present in the nucleus of mammalian cells, and was found to co-immunoprecipitate with snRNPs, the hyperphosphorylated form of RNA Pol II, and snRNAs, suggesting that PI4,5P2, and thus PIPKIα, may play a role in the processing of mRNA.
Accordingly, due to the importance of PIPKIα and the second messenger PI4,5P2 in numerous cellular pathways, identification of nuclear proteins that are directly modulated by these molecules was undertaken to better understand the control of nuclear functions, including protein expression and message regulation.